1. Field of the Invention
This invention relates generally to the testing of wood and specifically to testing un-dried or partially dried wood to evaluate its propensity to degrade upon drying, for example, by splitting.
2. Background Art
At a microscopic level, wood is composed of a collection of slender, tubular fibers aligned longitudinally along the axis of the trunk of the tree. The substantially longitudinal alignment of these fibers defines the grain of the wood.
Freshly felled lumber (henceforth termed "un-dried wood) contains considerable moisture. The total amount of stored water, or the "moisture content" of wood is typically expressed as a percentage of the weight of the stored water to the weight of the dry wood. The moisture content of un-dried wood, for example, may exceed 100%, indicating a weight of stored water greater than that of the dried wood. Alternatively, the amount of moisture stored in the wood may be expressed with respect to a "fiber saturation point" being defined as a point during the drying of wood, where the free water has first disappeared from the lumens of the fibers. Un-dried wood typically has a moisture content above the fiber saturation level.
The stored moisture in un-dried wood creates three problems: 1) it increases the weight of the wood, and 2) it increases the wood's susceptibility to decay, and 3) it creates the problem of spontaneous shrinkage of the wood when the wood is exposed to ambient, low humidity conditions in an assembled structure.
For this reason wood is typically dried, either by exposing the wood to the open air in loosely stacked racks, "air drying", or by drying the wood in the controlled atmosphere of a kiln, "kiln drying". Kiln drying represents a considerable improvement in drying speed over air drying, the latter which may require a month of drying time for boards up to an inch thick and more than a year for certain hardwoods or for thicker lumber. In contrast, kiln drying of most boards may be accomplished within a few weeks.
This improvement in drying time, in kiln drying, substantially decreases the lumber's final cost. Nevertheless, kiln drying is not costless and it is important to use the kiln efficiently. Efficient use of the kiln requires the careful selection of the kiln drying speed: if the drying speed is too slow, the total cost of the drying procedure will be unnecessarily increased, if the drying speed is too fast, however, undue checking and warpage of the wood will occur.
As the wood is dried it shrinks, but not equally along and across its grain. The alignment of wood's fibers makes wood an anisotropic material, that is, one which has varying physical properties along its different axes. Specifically, wood shrinks a proportionally greater amount in the transverse direction, perpendicular to the grain, also its direction of least tensile strength. This combination of factors creates the potential for checks as the wood is dried, that is, cracks or splits along the grain. Extensive checking along greater than two axes is termed honeycombing.
Splitting, honeycombing, and checking of dried wood all degrade the lumber, both aesthetically, and by reducing the strength of the wood. Even so, typically, a certain percentage of the lumber in a properly operated kiln will exihibit drying induced degradation. This is because of variation in the propensity of different pieces of lumber, even of the same wood type, to degrade upon drying, and because of an inability to determine this propensity to degrade when the wood is in the un-dried state.
The same problems apply to partially dried lumber, that is, lumber that has had some drying but not so much that the degradation is apparent. Thus the term "un-dried", should be understood to include not only fresh cut lumber but lumber that has not dried to the point where degradation occurs.
Of particular concern with regard to drying induced degradation is a condition called "wetwood". Wetwood is an abnormally high concentration of water in un-dried lumber thought to be caused by the operation of bio-deteriorants, e.g. anaerobic bacteria. Although it may be identified by a translucent or water soaked appearance, and a sour or rancid odor, generally, wetwood cannot be detected until during or after the drying process when it results in checking or splitting or honeycombing of the dried lumber. Importantly, un-dried wetwood may not exhibit significant differences in strength from normal wood not prone to degrading.
Wetwood and other such conditions that render a piece of wood "hard to dry" increase the expense of kiln dried lumber, both through the lost value of the degraded lumber and through the cost of the kiln space wasted in drying this lumber. To a lesser extent, the incidence of hard to dry lumber may also increase the cost of kiln drying by encouraging slower than necessary drying times for the remainder of the lumber.